This study is on a strain of Amoeba proteus, which became spontaneously infected with a large number of rod-shaped bacteria (60,000 - 150,000 per amoeba). The infective bacteria were initially harmful to the host amoeba, and were regarded as parasites. However, within a few years of infection (less than 1000 amoeba generations), the bacteria became not only harmless but essential to the survival of the hosts. Thus intracellular parasites changed to necessary symbionts within an observable period of time, the first recorded instance of its kind. The overall objectives of the project are to study the structural and metabolic relationships between the host cells and the infective organisms to elucidate the basis of their mutual dependence. The ultrastructural studies of the infected amoebae have been almost completed, and the dependence of host amoebae to the infective bacteria is being studied. We shall also study the kinetics of infection and the development of amoeba's dependence in detail after infecting normal amoebae experimentally. Such experimentally infected amoebae will be examined under the electron microscope to study the structural relationship. In addition, the infected amoebae and bacteria will be studied for their mutual metabolic dependence by various biochemical methods. The results are expected to contribute considerably to our understanding in several areas of cell biology, such as intracellular symbiosis, origin of eukaryotic cell organelles, and cell variation and divergence.